In prior art systems, the hook switch function is typically implemented between the telephone line and the diode bridge using a mechanical connection, such as a relay or the switch hook itself, as shown in FIG. 2. The hook switch relay can be electro-mechanical or optical, but in prior art configuration it must be able to switch current flowing in either direction. This approach is used so that the hook switch relay contact can be bridged by a resistor in series with a capacitor in order for telephone ringing signals to be capacitively coupled into the line circuit while the switch hook contact remains open (BC on-hook condition).
It is desired to replace the mechanical switch hook to alleviate cost and/or size reductions. Any such replacement must handle the power variations and polarity reversals which occur on telephone lines.